The present invention relates to a process for the production of nanoparticles or nanostructured particles using an emulsion method, particles being produced through targeted coalescence of at least two miniemulsions.
The prior art discloses a large number of production processes and uses of nanoparticles where solid or colloidal particles with a particle diameter of less than 1 μm can be referred to as nanoparticles. Nanoparticles can, for example, be composed of inorganic or polymeric material and in many cases have an average particle size of from 1 to 1000 nm. For the production of inorganic particles, the sol-gel process and microemulsion technology, for example, are known. Polymeric nanoparticles can be produced, for example, by emulsion polymerization. The targeted formation and structuring of nanoparticles is of particular interest for achieving particular properties of the nanoparticles for highly specialized applications.
Emulsions is the term used to refer to finely distributed mixtures of at least two liquids which are not homogeneously miscible with one another. One example is the mixture of oil and water. A liquid forms the so-called inner or disperse phase, which is present distributed in the form of small droplets in the second liquid, the so-called outer or continuous phase. Important constituents of emulsions are surface-active substances, so-called surfactants or emulsifiers, which facilitate the formation of the droplets and counteract demixing (phase separation). A distinction is made between oil-in-water emulsions (O/W emulsion), in which droplets of the nonpolar phase (for example oil droplets) are present in the continuous polar phase (for example water phase), and correspondingly water-in-oil emulsions (W/O emulsion), which are also called inverse emulsions. Emulsions should in most cases remain stable for a certain period and under certain conditions (e.g. temperature, pH range).
In conventional emulsions (macroemulsions), the drop sizes in the disperse phase are nonuniform. The drop sizes in macroemulsions are in most cases between 100 nm and 1 mm. Macroemulsions are thermodynamically unstable and often separate within a relatively short time. The term miniemulsion refers to a thermodynamically unstable emulsion, where the disperse phase is present in very finely distributed droplets with an average droplet diameter of <10 μm, in particular <5 μm. Miniemulsions are obtained, for example, through shearing with a high energy input starting from two (or more) immiscible liquids and one or more surface-active substances (surfactant, emulsifier). The droplets of a miniemulsion can be kept stable under certain conditions over a certain period, meaning that the production of particles in miniemulsions can take place through the melting of various droplets. The required energy input (for example in a shear process) for the production of miniemulsions can take place, for example, through ultrasound treatment or through use of a high-pressure homogenizer. Accordingly, miniemulsions in which a solid, for example in the form of nanoparticles, is present in the disperse phase, are referred to as minisuspoemulsions.
Thermodynamically stable emulsions, which constitute a special case and only exist in particular composition ranges in the water-oil-emulsifier phase system, are referred to as microemulsions. They form spontaneously and are often transparent in appearance. They mostly comprise a high fraction of surfactant and, moreover, in most cases a further surfactant, a cosurfactant. Mixing two microemulsions with different reactants in the disperse phase results in material exchange between the disperse phases and thus in the reaction without the emulsion drops having to be coalesced with the input of energy.
Miniemulsion processes for building up structured nanoparticles, for example with a core/shell structure are described in the prior art. The publication by K. Landfester (Adv. Mater. 2001, 13, No. 10, 17 May, 2001) describes the production of miniemulsions and the use of miniemulsions in the synthesis of nanoparticles and encapsulated nanoparticles. The synthesis of nanoparticles can take place, for example, with the help of miniemulsified molten salts or via the polymerization of a miniemulsified monomer. However, an option for the targeted coalescence of miniemulsion drops is not described here. A disadvantage of the method described here is that in each case one drop is reformed to give one particle, as a result of which the drop size has a clearly limiting influence on the size of the resulting particle.
WO 2008/058958 describes the production of core/shell particles, where an outer layer is applied to solid nanoparticles dispersed in a minisuspoemulsion by, in an emulsion process using an emulsion, converting a precursor substance dissolved in the disperse (preferably aqueous) phase in the disperse phase and thereby applying it to the dispersed nanoparticles.
The production of spherical inorganic nanoparticles through precipitation in a 2-emulsion method using microemulsions is known. Lee et al. (J. European Ceramic Society 19, 1999) describes the synthesis of spherical ZrO2 microparticles where two inverse microemulsions, which comprise precursor substances or reactants in the aqueous disperse phase, are mixed and reacted.
The coalescence, described in the prior art, of miniemulsions, induced by ultrasound treatment or rotor-stator systems (Ultra-Turrax) have the disadvantage that the coalescence of the emulsion droplets cannot be controlled. Moreover, an ultrasound treatment is rather unsuitable for a large-scale application since the ultrasound method is difficult to handle for industrial processes on a relatively large scale. The effect of ultrasound is generally locally limited and often leads to bimodal particle size distributions. The use of microemulsions is limited to narrow special cases and, moreover, has the disadvantage that large amounts of surfactant and cosurfactant contaminate the resulting product.